The development of the interaction between the beta-adrenergic excitatory mechanism activated by sympathetic neurotransmitter (isoproterenol is used because of its potency) and the muscarinic inhibitory mechanism activated by vagal transmitter, acetylcholine, is studied in the ventricle of hearts from embryonic and hatched chicks. The results obtained in our experiments support the cyclic AMP hypothesis, namely, that the excitatory effects of isoproterenol on Ca2 ion-dependent action potentials and contractions in the ventricle are associated with accumulation of cyclic AMP. Acetylcholine, which has no effect by itself on ventricular function in the embryo, nevertheless inhibits the effects of isoproterenol on cyclic AMP accumulation, Ca2 ion-dependent action potentials and contractions. After hatching, ACh inhibits cyclic AMP accumulation, Ca2 ion-dependent action potentials and contractility in the absence of beta-adrenergic agonist. In order to test the hypothesis that cyclic AMP is a focal point for the regulation of cardiac activity by autonomic neurotransmitters, the effects of the transmitters on adenylate cyclase and on phosphodiesterase ought to be determined. Such experiments are especially important in the evaluation of how the regulation of AMP metabolism may change during ontogenesis.